Multilateration techniques are known in the art. Such techniques may be used to locate the source of a radio transmission based upon differences in time of arrival (TOA) of radio signals at multiple receivers of known position. Such multilateration systems are illustrated, for example, in Heldwein et al. U.S. Pat. No. 4,229,737, issued Oct. 21, 1980; Jandrell, U.S. Pat. No. 5,365,516, issued Nov. 15, 1994, and Drouilhet Jr. et al. U.S. Pat. No. 5,570,095, issued Oct. 29, 1996, all of which are incorporated herein by reference.
Secondary Surveillance Radar (SSR) systems are used to track almost all commercial and many general aviation aircraft. An SSR system may send an interrogation signal to an aircraft in range of the radar on a 1030 MHz carrier frequency. All aircraft transponders which receive an appropriate interrogation reply to the SSR system on a 1090 MHz carrier frequency. In addition to SSR interrogations, TCAS (Traffic Alert Collision Avoidance System) units interrogate on a 1030 MHz carrier frequency for the purpose of collision avoidance.
Multilateration is a cooperative surveillance technique for aircraft equipped with Air Traffic Control Radar Beacon System (ATCRBS), Mode S, or Automatic Dependent Surveillance Broadcast (ADS-B) transponders described in U.S. Pat. No. 5,570,095, incorporated herein by reference. Prior Art multilateration systems utilize the 1090 MHz reply signals to perform a TDOA (Time Difference of Arrival) calculation to determine the origin of the transmission.
A plurality of receivers at different locations are used to receive the 1090 MHz reply signal. Each receiver used in the multilateration system utilizes a clock that is synchronized to a common time base. GPS (Global Positioning System) time, for example, may be used as a common time base.
When a 1090 MHz transponder reply transmission is received at a receiver, the message is time stamped and sent to a central location (via radio or hard-wire network) where the information gathered by all receivers is used to compute the origin of the transmission based upon the difference in propagation time of the 1090 MHz signal from the airplane to various receivers.
As with any system, there are errors associated with multilateration. Each receiver will have inherent errors based on properties such as clock drift and system latency. When the data from all receivers is combined for a solution, these errors produce one overall system error.
One way to reduce such error is to utilize a reference transponder to reduce overall system error. The reference transponder broadcasts on a 1090 MHz carrier frequency and is located at a known position. Because the position of each receiver is known and the location of the reference transponder is known, the time difference between reception of the reference transponder signal can be calculated. A single receiver is then chosen as a starting point for the calibration.
Since multilateration calculations depend on the differences in time of arrival and not time of arrival itself, correction of the DTOA is sufficient to correct the system. The starting receiver can be selected without concern for its accuracy but rather by some other criteria such as reception rates. The actual time differences between TOAs are compared to the calculated values and the corrections made to each receiver to adjust for the errors in DTOA.
Such a technique requires the construction and installation of a reference transponder, antenna, and associated support equipment. Moreover, such a broadcast installation may require licensing by government authorities (e.g., FCC). It would be preferable to utilize passive (e.g., receive-only) equipment for multilateration, as such equipment installations may not require governmental licensing. Moreover, a passive solution may reduce the amount of equipment and thus reduce overall cost.
Prior art SSR systems transmit RF signals in a pattern as illustrated in FIG. 2. When an SSR transmits an interrogation signal, the main beam 21 produces side lobes 22 as a side effect. Transponders receiving these side lobe transmissions must be suppressed from transmitting a reply in order to prevent confusion between transponders responding to the main beam and the side lobes.
In order to suppress spurious replies from side lobes 22, the SSR transmits omni-directional SLS (sideband lobe suppression) pulse 12. SLS pulse 12 transmitted simultaneously with the SSR pulse at a power slightly greater than the power of the strongest side lobe, as illustrated in FIG. 2. When a transponder receives an interrogation, it compares the interrogation to SLS pulse 12. If SLS pulse 12 has a power greater than the interrogation, no reply is produced. SLS pulse 12 is transmitted with each interrogation.